Switching power converters generally comprise a control module for controlling a switch module to switch on and off so as to convert an input voltage into an appropriate output voltage to supply a load.
Normally, the control module is configured to control the on and off switching of the switch module based on a feedback signal indicative of the output voltage and a reference signal indicative of a desired value of the output voltage, so that the control module can regulate the output voltage in response to changes in the load. A negative feedback control loop is thus established. The reference signal is predetermined and generally provided by an internal reference generation circuit e.g. a bandgap reference circuit. The feedback signal can be provided by a feedback circuit configured to sense the output voltage of the power converter with a feedback coefficient. The negative feedback control loop is aimed to regulate the feedback signal to track the reference signal based on a difference between the feedback signal and the reference signal so as to maintain the output voltage at the desired value.
Typically, an error amplifier is utilized to conduct operation to the feedback signal and the reference signal so as to provide the difference between the feedback signal and the reference signal. Trans-conductance error amplifier is one type of the most often used error amplifiers, a trans-conductance of which can be determined by a ratio of a bias current provided to the trans-conductance error amplifier to a thermal voltage. The bias current is normally substantially constant and the thermal voltage is also constant. Thus, the trans-conductance is substantially constant. For a switching power converter using a trans-conductance error amplifier, a band width of the negative feedback loop may decrease with increase in the desired value of the output voltage. This is because the band width is in direct proportion with the feedback coefficient of the feedback circuit. Providing that the reference signal is predetermined, should the desired value of the output voltage decrease, the feedback coefficient should be decreased accordingly. However, in practical application, it is desired that the band width of the negative feedback loop does not decrease with the increase in the desired value of the output voltage.